O R I G I N A L A R T I C L E
The shallow thermal regime of Devils Hole,
Death Valley National Park
Mark B. Hausner,1,2 Kevin P. Wilson,3 D. Bailey Gaines,3 Francisco Suarez,2 and Scott W. Tyler1
Abstract
Devils Hole, a fracture in the carbonate aquifer underlying the Death Valley Regional Groundwater Flow
system, is home to the only extant population of Devils Hole pupfish (Cyprinodon diabolis). Since 1995,
the population of C. diabolis has shown an unexplained decline, and a number of hypotheses have been
advanced to explain this. Here, we examine the thermal regime of Devils Hole and its influence on the
pupfish population. We present a computational fluid dynamic (CFD) model of thermal convection on
the shallow shelf of Devils Hole, which provides critical habitat for C. diabolis to spawn and forage for
food. Driven by meteorological data collected at Devils Hole, the model is calibrated with temperature
data recorded in the summer of 2010 and validated against temperatures observed on the shallow shelf
between 1999 and 2001.The shallow shelf experiences both seasonal and diel variations in water tem-
perature, and the model results reflect these changes. A sensitivity analysis shows that the water tem-
peratures respond to relatively small changes in the ambient air temperature (on the order of 1 8C), and a
review of local climate data shows that average annual air temperatures in the Mojave Desert have
increased by up to 2 8C over the past 30 years. The CFD simulations and local climate data show that
climate change may be partially responsible for the observed decline in the population of C. diabolis that
began in 1995.
Keywords: convection, climate change, computational fluid dynamics, Devils Hole, distributed temperature
sensing
Introduction
[1] The endangered Devils Hole pupfish
(Cyprinodon diabolis; Wales 1930) occurs only
in the geothermally influenced Devils Hole eco-
system (36.42 N, 116.28 W), a groundwater-
filled fracture in the carbonate aquifer under-
lying Ash Meadows National Wildlife Refuge
in southern Nevada, USA. With a surface area
of just 50 m2, Devils Hole comprises a shallow
shelf (average depth ,0.35 m) perched adjacentto a pool that extends more than 130 m
deep. Over the past 40 years, the population
of C. diabolis has varied significantly, declining
as groundwater mining depleted the water level
in the ecosystem and then recovering in the
1970s as the water level increased (although
it never reached prepumping levels) after the
cessation of pumping (Andersen and Deacon
2001). In the mid-1990s, the population of
C. diabolis began a second decline, with no
immediately apparent cause (Riggs and Deacon
2002). Despite ongoing management of the eco-
system, the population remains at a reduced
level, and the post-1995 population decline
Limnology and Oceanography: Fluids and Environments 3 (2013): 119138 DOI 10.1215/21573689-2372805
A 2013 publication of the Association for the Sciences of Limnology and Oceanography
1Department of Geological
Sciences and Engineering,
University of Nevada, Reno,
Reno, Nevada 89557 USA
2Department of Hydraulic and
Environmental Engineering,
Pontificia Universidad Catolica
de Chile, Santiago, Chile
3Death Valley National Park,
Pahrump, Nevada 89048 USA
Correspondence to
Mark B. Hausner,
[email protected]
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remains unexplained. A number of hypotheses have
been advanced to explain this decline, including
inbreeding depression (Wilcox 2001), shifts in solar
radiation and allochthonous inputs (Wilson and Blinn
2007), changes in sediment dynamics (Lyons 2005) and
the algal community (Riggs and Deacon 2002), and the
loss of a prey species from the ecosystem (Herbst and
Blinn 2003). Threloff and Manning (2003) posited that
increases in water temperature on the ecologically criti-
cal shallow shelf may be partially responsible for the
population decline.
[2] The approximately 17 m2 shallow shelf com-
prises a boulder perched between the walls of the frac-
ture and provides the best C. diabolis spawning habitat
in the system (Andersen and Deacon 2001), as well as a
substrate for the growth of the benthic community
(Wilson and Blinn 2007). With a high surface area
tovolume ratio, water temperatures on the shelf vary
on both a daily and a seasonal basis. Devils Hole pupfish
use the shallow shelf for spawning and foraging on a
daily basis (James 1969; Baugh and Deacon 1983).
Although pupfish spawn year round (Chernoff 1985),
the population of C. diabolis normally follows a seasonal
cycle (Fig. 1A), with the greatest annual increase in
population observed between May and July (Riggs and
Deacon 2002). Regular population surveys in Devils
Hole generally count individuals 1718 mm or larger,
although smaller fish are occasionally included in the
surveys when they are observed. Given the observed
growth rates of C. diabolis, it takes 23 months for
the fish to reach this length (James 1969); this lag time
between recruitment and population surveys implies
that the majority of the annual recruitment occurs in
March and April.
[3] The life cycle of Cyprinodon spp. (or pupfishes)
is strongly controlled by water temperatures, particu-
larly during life stages associated with reproduction.
In a study of Amargosa pupfish (Cyprinodon nevadensis
nevadensis, closely related to C. diabolis), Shrode
and Gerking (1977) delineated increasingly narrow
ranges of acceptable temperatures for normal activity
(1238 8C), viable oogenesis (2436 8C), and successful
reproduction (2630 8C), defined as .50% survival.
Although spawning activity occurs year-round, the sea-
sonal population cycle indicates that annual recruitment
of C. diabolis is controlled by other factors. The food
web in Devils Hole is dynamic, dominated in the sum-
mer by primary productivity but changing to an alloch-
thonous-driven system when the low winter sun and the
canyon walls surrounding Devils Hole limit direct sun-
light. Annual recruitment of C. diabolis begins in March,
coinciding with the springtime increase in both alloch-
thonous carbon input and solar radiation (which drives
increased primary productivity; Wilson and Blinn
2007). After the end of April, mean water temperatures
rise sharply and remain relatively constant between May
and August (Fig. 1C). Whereas other environmental fac-
tors influence the viability of pupfish eggs and larvae
(e.g., dissolved oxygen, hydrogen sulfide, and preda-
tion), suitable water temperatures and available food
are both necessary for successful recruitment. The
beginning of the annual recruitment period occurs
when both allochthonous carbon inputs and primary
productivity begin increasing in the spring, and this
period appears to end when warm water temperatures
prevent successful egg development.
[4] This article presents a computational fluid
dynamic (CFD) model of thermal processes in the
water column over the shallow shelf of Devils Hole
and compares simulated temperatures with both his-
torical records and temperatures observed using a
fiber-optic distributed temperature sensor (DTS). The
model is used to examine the circulation of water within
the system and to consider the potential impacts of cli-
mate change on the ecosystem. By integrating CFD
modeling with field observations, we examine the
hypothesis that climate change can affect the water tem-
peratures on the shallow shelf of Devils Hole strongly
enough to affect the recruitment of C. diabolis. The util-
ity of hydrodynamic modeling as a tool for evaluating
changing ecosystems is demonstrated, and the model
allows recommendations for future research in both
Devils Hole and the American Southwest.
Methods
The Devils Hole System
[5] The ecologically critical shallow shelf of Devils Hole
(Fig. 2) is approximately 5.8 m long (northsouth) with
a width that varies between 2 and 3 m (eastwest).
Because of the orientation of the shelf within the narrow
120 Limnology and Oceanography: Fluids and Environments 3 (2013)
A 2013 publication of the Association for the Sciences of Limnology and Oceanography / e-ISSN 2157-3689
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